Analog/digital converters are notably used in image sensors to convert an analog signal representative of the illumination sensed by a photodiode of a pixel into a digital word for storing in memory and processing in digital circuits. In the majority of image sensors fabricated using CMOS technology, an analog/digital converter is generally assigned to each column of pixels of the sensor and the conversions are carried out simultaneously for the pixels of the same row.
One type of analog/digital converter typically used is a ramp-type analog/digital converter, also known by those skilled in the art under the acronym SS-ADC (“Single-Slope Analog to Digital Converter”). The structure and the operation of such a ramp-type analog/digital converter is well known to those skilled in the art and the latter will, for example, be able to refer as required to the European Patent application published under the number EP 2456075 and/or to the article by G. G. Storm et al.: “Continuous Time Column Parallel Readout for CMOS Image Sensor”, 2007 International Image Sensor Workshop, Jun. 7, 2007-Jun. 10, 2007, pages 58-61.
Such ramp-type analog/digital converters are notably used owing to their compactness, to their low power consumption and to their low noise.
Currently, there is a requirement for high-speed image sensors having a large matrix of pixels, high data rates and capable of multiple exposures.
In a corresponding manner, there is also a growing desire to increase the resolution of analog/digital converters going, for example, from 10 to 12 bits.
However, this increase in resolution comes with limitations notably when it is desired to use ramp-type analog/digital converters. This is because an increase in resolution of 2 bits leads to a duration of the ramp multiplied by 4, which may increase by the same factor the period of time for carrying out the analog/digital conversion.